Buggy code:
public class InterpreterEngine extends Walker { private final ClassTable classTable = new ClassTable ( ) ; private ClassInfo currentClassInfo ; private List < NId > idList ; private Token operatorToken ; private List < NExp > expList ; private Instance expEval ; private Frame currentFrame ; private ClassInfo objectClassInfo ; private BooleanClassInfo booleanClassInfo ; private IntegerClassInfo integerClassInfo ; private StringClassInfo stringClassInfo ; public void visit ( Node node ) { } public void printStackTrace ( ) { } private List < NId > getParams ( NIdListOpt node ) { } private Token getOperatorToken ( NOperator node ) { } private Instance getExpEval ( Node node ) { } private List < NExp > getExpList ( NExpListOpt node ) { } private Instance execute ( MethodInfo invokedMethod , Frame frame , Token location ) { } @ Override public void caseFile ( NFile node ) { } @ Override public void inClassdef ( NClassdef node ) { } @ Override public void outClassdef ( NClassdef node ) { } @ Override public void caseMember_Field ( NMember_Field node ) { } @ Override public void caseMember_Method ( NMember_Method node ) { } @ Override public void caseMember_Operator ( NMember_Operator node ) { } @ Override public void caseMember_PrimitiveMethod ( NMember_PrimitiveMethod node ) { } @ Override public void caseMember_PrimitiveOperator ( NMember_PrimitiveOperator node ) { } @ Override public void inIdList ( NIdList node ) { } @ Override public void caseAdditionalId ( NAdditionalId node ) { } @ Override public void caseOperator_Plus ( NOperator_Plus node ) { } @ Override public void caseOperator_Eq ( NOperator_Eq node ) { } @ Override public void caseStm_VarAssign ( NStm_VarAssign node ) { } @ Override public void caseStm_FieldAssign ( NStm_FieldAssign node ) { } @ Override public void caseStm_While ( NStm_While node ) { } @ Override public void caseStm_If ( NStm_If node ) { } @ Override public void caseStm_Return ( NStm_Return node ) { } @ Override public void caseStm_Call ( NStm_Call node ) { } @ Override public void caseStm_SelfCall ( NStm_SelfCall node ) { } @ Override public void caseExp_Is ( NExp_Is node ) { } @ Override public void caseExp_Eq ( NExp_Eq node ) { Instance left = getExpEval ( node . get_Exp ( ) ) ; Instance right = getExpEval ( node . get_AddExp ( ) ) ; if ( ( left == null ) || ( right == null ) ) { if ( left == right ) { this . expEval = this . booleanClassInfo . getTrue ( ) ; } else { <START_BUG> this . expEval = this . booleanClassInfo . getTrue ( ) ; <END_BUG> } } else { MethodInfo invokedMethod = left . getClassInfo ( ) . getMethodTable ( ) . getMethodInfo ( node . get_Eq ( ) ) ; Frame frame = new Frame ( this . currentFrame , left , invokedMethod ) ; frame . setParam ( right ) ; this . expEval = execute ( invokedMethod , frame , node . get_Eq ( ) ) ; } } @ Override public void caseExp_Isa ( NExp_Isa node ) { } @ Override public void caseAddExp_Add ( NAddExp_Add node ) { } @ Override public void caseLeftUnaryExp_Not ( NLeftUnaryExp_Not node ) { } @ Override public void caseTerm_New ( NTerm_New node ) { } @ Override public void caseTerm_Field ( NTerm_Field node ) { } @ Override public void caseTerm_Var ( NTerm_Var node ) { } @ Override public void caseTerm_Num ( NTerm_Num node ) { } @ Override public void caseTerm_Null ( NTerm_Null node ) { } @ Override public void caseTerm_Self ( NTerm_Self node ) { } @ Override public void caseTerm_True ( NTerm_True node ) { } @ Override public void caseTerm_False ( NTerm_False node ) { } @ Override public void caseTerm_String ( NTerm_String node ) { } @ Override public void caseCall ( NCall node ) { } @ Override public void caseSelfCall ( NSelfCall node ) { } @ Override public void caseExpList ( NExpList node ) { } @ Override public void caseAdditionalExp ( NAdditionalExp node ) { } public void integerPlus ( MethodInfo methodInfo ) { } public void stringPlus ( MethodInfo methodInfo ) { } public void objectAbort ( MethodInfo methodInfo ) { } public void integerToS ( PrimitiveNormalMethodInfo primitiveNormalMethodInfo ) { } public void stringToSystemOut ( PrimitiveNormalMethodInfo primitiveNormalMethodInfo ) { } }

Fix:
this . expEval = this . booleanClassInfo . getFalse ( ) ;